1. Field of the Invention
This invention relates to intensity calibration targets and methods employed in imaging systems, which targets and methods are particularly useful in systems having a plurality, such as a pair of imaging arrays for scanning both sides of a document.
2. References
Acquisition of color, black and white, and shades of gray image information, adaptable for generating copies or electronic processing directly from printed original documents, has become greatly desirable as an adjunct to electronic document generation. An important reason for this desirability is the subsequent capability of manipulation of the electronically stored information for editing, compiling and using the information in forms other than those in which it was originally available. While such manipulation is, available for image information produced originally or otherwise available in an electronic format, it is desirable to have a similar capability for image information not so available. Accordingly, it is desirable to have automated image information input capability, which, coupled with available output devices, renders possible functions such as simplex and duplex copying, image rotation, cropping, editing, and the like, without the requirement of troublesome mechanical manipulation of originals and copies.
A number of document handling systems utilize image input devices for such purposes as archiving, printing and transmitting images. Typical applications include facsimile transmission, document reproduction, digital copying, inputting images into a database, and optical character recognition. Specifically a number of known systems utilize a linear array of solid-state light detecting devices to generate an image, linear area by linear area, until the entire image of, for example, a side of a sheet of paper is acquired.
In such devices, constant velocity relative movement, between the linear array and the document to be imaged, allows the linear array to scan the face of the document. This is usually achieved in one of two ways. Either the linear detector array may be held stationary and the document moved at a constant velocity past the linear array, or, alternatively, the document may be held stationary and the linear array scanned across the face of the document, carried by a constant velocity transport.
Each of the elements in the linear array receives one pixel of intensity information from the corresponding part of the image in the form of light from an illumination lamp reflected by that part of the image, and focused by an appropriate lens structure. In response, each of the elements outputs an electrical signal whose magnitude is indicative of the intensity of light falling on the element.
A primary problem associated with electronic input scanners is a periodic requirement for calibration of the sensor arrangement and its electronics and related optical imaging components. Because a large number of photosensitive elements comprise the scanning array, uniformity of response is of value for acceptable imaging quality.
The problem thus arises to determine what level of electrical output signal corresponds to white and black image portions. Black signal levels may be measured by simply turning the illumination lamp off. If the level of electrical output signal corresponding to a white image portion was always the same, the problem of determining what level of electrical output signal corresponds to a white image portion would be relatively simple to solve, by simply hard wiring a circuit which provides the desired response. However, a white image portion may cause a photodetector to emit a wide range of output voltages, depending upon numerous factors, such as ambient light, line voltage variations, pixel to pixel variations in the detector array, lamp age and manufacturing variations, optics, dirt, and other factors. Frequent calibration is required against a target having a known reflectance value.
In U.S. Pat. No. 5,280,368 to Fullerton, there is disclosed a dual purpose calibration/baffle member, which in a first position serves as a paper baffle along the sheet feeding path to the scanning station to support sheets at a first scanning element, and in a second position, supports a calibration target at a position for detection by the first scanning element. The calibration/baffle member is mounted on the input scanner frame for movement between the first and second positions. The baffle member is guided with pivoting movement for the purpose of calibration to a position where a calibration target is within the field of view of the fixed scanning element.
In U.S. Pat. No. 4,429,333 to Davis, a calibration strip cut from a sheet of ethylene propylene or other uniformly white material is disclosed. The calibration strip is pressed against the platen glass on the same side as the document. Prior to commencement of scanning, a dark calibration reference is established by scanning with the illumination off. Next, the illumination is turned on, and the scanner carriage is driven to a calibration position where the scanner views the calibration strip. At that time, the scanner is calibrated to produce output signals based on the known reflectivity of the calibration strip.
In Buchar, U.S. Pat. No. 4,967,233, it is indicated that the scanning element is rotated out of the scanning position to view a calibration target. More particularly, the scanning element is rotated about an axis transverse to the direction of paper travel through the scanning station, and parallel to the paper path, with the axis through the scanning element.
U.S. Pat. No. 4,574,316 to Wilman et al. discloses a document scanner unit which rotates into at least one other scanning position to receive light reflected from a remote source.
U.S. Pat. No. 4,464,681 to Jacobs et al. discloses an optical scanning system comprising a linear photodiode array which can be adjusted in position to view an optical test pattern. U.S. Pat. No. 4,605,970 to Hawkins discloses a calibration arrangement which moves an optical scanning head assembly from a reference location into a testing position to view an optical test pattern. U.S. Pat. No. 4,706,125 to Takagi discloses an image reading device comprising an integrated image reading unit and an optical sensitivity checking member which concurrently translate in unison from an inoperative position into an operative position during the scanning of an original.
U.S. Pat. No. 4,806,977 to Mizutani et al. discloses a movable carriage housing for a scanning-type optic apparatus wherein a rack and pinion arrangement allows an upper body portion apparatus to pivot outwardly to expose a transfer station and scanning head for maintenance.
It is desirable to provide a scanning device for scanning duplex documents, for example, original documents having image information on both sides, for simplex documents having image information only on a single side, and for material not adaptable to be passed through sheet handling devices. In the past, this feature has been achieved in input scanners in a variety of ways, for example, there is disclosed in U.S. Pat. No. 4,536,077 to Stoffel, an arrangement provided with an optical system to direct light reflected from a first side of the document to a single scanning array, while the document is moving past a first position, and subsequently directing light from the second side of the document to the scanning array when it has reached a second position.
A disclosure entitled “Automatic Duplex Document Electronic Scanning” by Richard E. Smith, and published in the Xerox Disclosure Journal, Vol. 8, No. 3, May/June, 1983 at page 263, discloses both side scanning of a document with two spaced apart scanning arrays arranged on opposite sides of a document path, and platen scanning by a movable carriage supporting one of the arrays. All the patents and publications cited hereinabove are incorporated herein by reference.
Another solution to the calibration problem involves the reading of the intensity of light from a selected portion or portions of a white calibration target. In one arrangement the calibration target is on the same side of the platen glass as the document to be imaged. In accordance with the invention, it is noted that because contact between the calibration target and the glass may result in uneven intensity at one or more points, an air gap is introduced between the calibration target and the platen glass.